1. Field of the Invention
The present invention is in the field of devices for the automatic guidance of welding torches; more specifically, this invention is in the field of devices for the guidance of welding torches for effecting welded seams in narrow grooves.
2. Description of the Prior Art
In welding operations generally, it is sometimes necessary to join relatively thick pieces of metal in such fashion that the original juxtapostion of the separate pieces is close with respect to size of the torch, the size of the filler wire, the size of the seam, or any of these.
These are devices known which have approached this problem. For instance, Garfield et al., in U.S. Pat. No. 3,826,888, show an arc-welding torch for welding seams in deep narrow gaps; the configuration of the contact-tube tip causes the weld wire to assume a curve as it is fed out of the end of that tip. The curve in the wire causes it to bear generally into the corner of the joint as the torch moves along. However, Garfield et al. have no provision for guidance of the torch, and have to assume that the joint is substantially uniform. While this assumption is valid in many cases, there are a number of situations where the nature of the joint makes such an assumption an improper one, such as e.g., where the joint comprises compound angles, variations in width, assembly tack welds, and the like.
One solution to the problem of non-uniformity of the joint is found in Ujiie, U.S. Pat. No. 3,746,833, where a narrow gap is filled in one pass by the use of three weld wires in effecting the seam. The three wires permit the use of relatively thin pieces, while at the same time feeding a large net amount of filler into the joint. By the process described by Ujiie, two of the thin wires fill the corners of the joint with ample allowance for imperfections and irregularities, and the third wire fills any spaces left between the first two wires. As with Garfield et al., Ujiie uses no guidance per se, relying on either a pre-programmed system or an assumption that the joint will not deviate more than the inherent ability of the three-wire method to accommodate.
Nelson et al., in U.S. Pat. No. 3,328,556, show one means of guidance of the torch in a process for narrowgap welding. A spring clip is provided, affixed to a contact tube carrying the filler wire; as the spring clip traverses the wall of the piece to be welded, irregularities in the wall are detected, and the contact tube is maintained at substantially the same distance from the wall. This method, however, relies on the assumption that irregularities in the wall are the same as those at the joint intersection, i.e., if there is a dip or bulge at the line where the spring clip follows, that dip or bulge will also occur at the joint interface.
It will be apparent to those skilled in the art that Garfield et al. and Ujiie do not guide the torch, and that Nelson et al. use a guidance system which may or may not cause the system to track the seam itself, as opposed to a reference line near, but not at, the joint.
In the case of arc-welding torches, there is also the problem of maintaining the filler wire discrete from the weld seam or the work until the wire is at the point where the weld is to be effected. Because the filler wire is necessarily at a different electrical potential from the work, either the wire must be insulated from the torch and permitted to touch the work only where desired, or the wire and torch, at the same potential, must be isolated from the work, with only the wire, as above, permitted to touch the work at the point of the weld. There are systems known which impart a bend to the filler wire before its approach to the weld seam, such that the end of the wire tends to sweep back and forth on the seam. This approach, however, necessarily means that the seam is welded with an overabundance of wire rather than only the proper amount, at the place where desired.
A futher problem which exists with attempts to solve the difficulties encountered in effecting welds in narrow gap is that of properly blanketing the weld area to prevent oxidation of the molten metal. Where a nozzle has been used to direct gas toward the weld area, the turbulence caused by the gas velocity tends to cause atmospheric oxygen to be entrained. Substituting a gas-diffusion system, comprising a gas passage in the torch and a sintered-bronze exit chamber near the end of the torch where the weld is effected, requires the use of uneconomically large amounts of inert gas, and further can cause the waste heat from the welding operation to be retained within the gas envelope. The retained heat then causes the apparatus to become hotter than is proper for efficient operation.